1. Field of the Invention
The present invention is directed to a cathode assembly including a target made of a soft magnetic material for use with a sputtering apparatus and, more particularly, to an apparatus and method for improving efficient use of the target.
2. Description of the Related Art
Sputter or physical vapor deposition (PVD) systems are well known and typically include a cathode assembly having a target made of a material to be deposited on a substrate, for example, a wafer. Typically, the cathode assembly of the sputtering apparatus is disposed in a vacuum chamber into which a gas, such as non-oxidizing argon, is introduced. In operation, power is supplied to the cathode, thus ionizing the gas.
The cathode assembly includes a magnetic field source that generates a magnetic field that focuses the plasma, thus causing a pool of ionized atoms to be available for acceleration towards the surface of the target. This bombardment of ionized atoms causes the metal atoms of the target to be sputtered from the target and deposited as a film on the substrate.
Notably, conventional magnetron sputtering or PVD of soft magnetic material (i.e., material easy to magnetize/demagnetize) results in inefficient use of the target material. This is due to the fact that the magnetic flux generated by a conventional magnetic field source causes a high density of ions to bombard the target in a confined area. More specifically, the magnetic field source is configured such that the magnetic field exhibits a cascading effect of narrow magnetic confinement which leads to formation of narrowing magnetic gaps, thus causing deep erosion in a relatively confined area. There is a progressive worsening of the erosion width to depth ratio, commonly referred to as erosion xe2x80x9ccusping,xe2x80x9d the longer the apparatus is operated.
For a standard cathode assembly 1 of a planar magnetron having a target 3 subject to magnetic field source 2, erosion occurs generally around the perimeter of the target 3 in a xe2x80x9cracetrackxe2x80x9d pattern 4 on surface 6, as shown in FIG. 1A. Magnetic field source 2 includes a plurality of inner magnets 7 (shown in dashed lines) having a first polarity and a plurality of outer magnets 8 having a second, opposite polarity (with the southern pole of magnets 8 being disposed generally adjacent a rear surface 9 of target 3), as shown in FIGS. 1A and 7A. With this magnet configuration, the magnetic flux lines form a family of curves 5 each having the shape of a tunnel and extending around the target generally in the racetrack pattern 4 to focus the plasma. As the magnetic field is observed closer to the target surface, the tangential, or in-plane, component of the magnetic field becomes infinitely smaller and approaches a point. It is this xe2x80x9cpointxe2x80x9d of the magnetic field that significantly contributes to erosion cusping. Notably, erosion cusping occurs at about the centerline 4A of the racetrack 4.
This phenomenon is shown more prominently in FIGS. 1B and 1C. FIG. 1B shows a portion of target 3 prior to sputtering, the target being made of a ferromagnetic material having a predetermined thickness (t) and having, for example, a generally rectangular shape. In FIG. 1C, target 3 is shown after approximately twenty kilowatt-hours of sputtering target 3 is shown to have undergone erosion cusping. The attendant erosion of target 3 is concentrated in a defined region 10 of the racetrack, and extends nearly entirely through the thickness (t) of the target. As a result, the target must be replaced even though a large majority of the target outside the cusp area is unutilized.
Other ferromagnetic target designs have been proposed with, at best, marginal improvement in the efficiency of target utilization over the above described system. For example, one known system utilizes slots formed entirely (or nearly entirely) through the target so as to avoid concentration of the erosion at a particular region. Although this known method can make a magnetic material behave like a nonmagnetic material, and thus improve utilization of the target, such a target typically exhibits xe2x80x9cfeatheringxe2x80x9d of the erosion near the discontinuities between the slots, thus generating particulate from the target structure after multiple hours of operation. As a result, the backplate to which the target is mounted may be exposed to ion bombardment, thus causing unwanted overheating/sputtering of the cathode assembly. Moreover, the target is cleaned prior to being installed for use and, when such slots are included, this cleaning process is made extremely difficult, with a higher risk of contaminates being left on the target. In addition, forming the slots makes such targets relatively difficult to manufacture and causes a significant portion of the target to be wasted.
As a result, the field of sputter deposition of soft magnetic materials is in need of an apparatus and method for distributing the density of ion bombardment on the associated target in such a way as to provide more uniform erosion of the target to increase the effective life of the target. The apparatus and method should avoid problematic erosion cusping while maximizing the amount of target material ultimately used, thus increasing system up-time by increasing the time between target changes and reducing the user""s end cost.
The present invention provides an apparatus and method for maximizing target utilization by ensuring that the ion density near the target surface is not concentrated at any particular point or region, thus avoiding the problem of erosion cusping. The cathode assembly preferably utilizes a magnetic field generator having conventional configuration, but with unconventional components including magnet assemblies that allow tuning of the magnetic field. More particularly, by tuning the magnitude of the tangential component (i.e., the component(s) of the magnetic field parallel to the target surface and most responsible for the extent of concentrated target surface erosion) of the magnetic field, ion concentration can be selectively minimized at a particular location, e.g., at the racetrack centerline where, as described previously, erosion cusping is most prominent. In addition to tuning the magnetic field, the preferred embodiment also requires a target that has a sculpted surface to facilitate erosion that is generally uniform transverse to the racetrack centerline.
According to one aspect of the preferred embodiment, a sculpted target for a sputtering apparatus includes a monolithic plate made of a soft magnetic material, the plate having a first surface and a center. In addition, the target has a sculpted section formed in the first surface. The sculpted section is generally recessed from the first surface and extends around the plate center in a racetrack configuration, wherein the racetrack has a concentric centerline. Notably, the sculpted section is generally symmetric about the centerline.
According to another aspect of the preferred embodiment, the sculpted section includes a plurality of generally adjacent trenches formed in the first surface, wherein an equal number of the trenches flank either side of the centerline. Moreover, each trench has an associated depth with the depths progressively increasing as the associated trenches are displaced generally further from the centerline.
According to a further aspect of the preferred embodiment, a cathode assembly includes a monolithic target having a first surface and a center region. In addition, a sculpted section is formed in the first surface, and the sculpted section is generally recessed from the first surface and extends around the center in a racetrack configuration. The racetrack has a concentric centerline, and the sculpted section preferably is generally symmetric about the centerline. A magnetic field generator is disposed adjacent to the target and produces a magnetic field having an in-plane component. The magnetic field generator is tuned so that a distribution of the magnitude of the in-plane component in a direction transverse to the centerline at a point along the racetrack is characterized by two peaks that have a generally equal magnitude.
According to yet another aspect of the preferred embodiment, a method of making a cathode assembly for sputtering a target made of a particular material includes the steps of using a target having a first surface to be sputtered and a second surface opposite the first surface, and then forming a sculpted portion in the first surface. Thereafter, the method includes disposing a magnetic field generator generally adjacent to the second surface to generate a magnetic field having an in-plane component, and thereafter tuning the magnetic field so as to avoid erosion cusping of the first surface during a sputtering operation.
According to another aspect of the method of the preferred embodiment, the magnetic field generator includes a plurality of magnet assemblies having a stack of elements, and the tuning step includes (1) selecting a combination of the elements, whereby the elements are selected from a group comprising: a magnet, an air gap, and a soft magnetic spacer, and then (2) measuring the magnitude of the in-plane component of the magnetic field for each point along a line transverse to the centerline to create a magnitude distribution associated with the line. Then, the selecting and measuring steps are repeated until the distribution includes two peaks.